1. Field of the Invention
The present invention relates to reflectors which are suitable for use in liquid crystal displays using external light, a front light, a backlight, etc., as a light source, and to liquid crystal displays using the reflectors. More specifically, the present invention relates to a reflector which exhibits good reflectance over a wide angle range and especially high reflectance in a reflection direction in a desired range, and to a liquid crystal display which uses the reflector so that it has a wide viewing angle and exhibits moderate directionality such that the display appears sufficiently bright when seen from a typical viewing area.
2. Description of the Related Art
Liquid crystal displays are commonly used as display units for mobile computers, etc., and reflective liquid crystal displays, which use external light as a light source, are one kind of liquid crystal displays which are commonly used because of their low power consumption. In addition, liquid crystal displays having a front light for obtaining extra light in addition to external light are also commonly used.
In such liquid crystal displays, external light incident on the display surface (from the observer side) or light emitted from a front light is reflected by a reflector and is emitted outside the display surface, so that a user can view an image which changes in accordance with the alignments of liquid crystal molecules in a liquid crystal layer.
In addition, liquid crystal displays having a backlight for obtaining extra light in addition to external light are also commonly used. In liquid crystal displays having a backlight, a semi-transmissive reflector is used in order to reflect external light and to pass light emitted from the backlight.
The inventors have performed various investigations with respect to the relationship between the shape of the surface of a reflector (the shape of the surface closer to a display surface) and reflection characteristics of the reflector.
When a reflector having a flat, specular surface is used, the reflector exhibits extremely high reflectance at a specific reflection angle determined in accordance with an incidence angle. However, a reflection-angle range in which high reflectance is obtained is extremely narrow. Thus, a reflector having high directionality such that a viewing area from which the reflector appears bright is narrow is obtained. In addition, visibility is degraded due to so-called back reflection, that is, reflection of a light source, an observer""s face, etc. in a display surface.
Accordingly, several techniques have been suggested in which concave portions having shapes like parts of a sphere, grooves, or irregular concavities and convexities are formed over the surface of a reflector in order to obtain good reflectance over a wide range. According to these techniques, reflection characteristics can be made such that the reflector appears bright over a wide viewing area.
FIG. 9 shows a reflector in which a plurality of concave portions each shaped like a part of a sphere are formed on the surface of the reflector. With reference to FIG. 9, a reflector 51 is constructed by forming a plate-shaped resin base member 53 (a base member of the reflector) made of a photosensitive resin layer, etc., on a substrate 52 made of glass, etc., and forming a plurality of concave portions 54 over the surface of the resin base member 53. The inner surfaces of the concave portions 54 are shaped like a part of a sphere, and the concave portions 54 are formed continuously so that the concave portions 54 overlap one another. In addition, a reflective film 55 formed of a thin layer of aluminum, silver, etc. is formed on the concave portions 54 by vapor deposition, plating, etc.
The concave portions 54 are formed such that the depth thereof varies in the range of 0.1 xcexcm to 3 xcexcm, and are irregularly arranged such that the pitch between the concave portions 54 varies in the range of 5 to 50 xcexcm. In addition, the inner surface of each concave portion 54 is shaped like a part of a single sphere, and an inclination angle thereof is set in the range of xe2x88x9218xc2x0 to +18xc2x0.
The term xe2x80x9cdepth of a concave portionxe2x80x9d used herein means the distance between the substrate surface of a reflector and the deepest point of a concave portion, and the term xe2x80x9cpitch between adjacent concave portionsxe2x80x9d used herein means the distance between the central points of adjacent concave portions, which have a circular shape as seen in a plan view. A surface as used herein is essentially a flat surface that disregards the minute irregularities (e.g. relatively microscopic crevasses or projections) present in almost every physical layer. Such a flat surface includes, for example, the substrate surface in which the concave portions are non-existent or are completely filled in.
In addition, xe2x80x9cinclination anglexe2x80x9d means an angle of a tangential line at an arbitrary point on the inner surface of the concave portions 54 relative to the substrate surface in a specific vertical section.
The reflector 51 has reflection characteristics similar to those of a comparative example (see FIG. 6), which will be described below. FIG. 6 is a graph showing the reflection characteristics in the case in which an incidence angle is 30xc2x0, where the vertical axis shows reflectance (reflection intensity) and the horizontal axis shows a reflection angle.
With reference to FIG. 10, an incidence angle is defined as an angle xcfx890 between the normal H of the reflector 51 (substrate surface) and incident light J. In addition, a reflection angle is defined as an angle xcfx89 between the normal H and reflection light K on a plane including the normal H and the incident light J. In addition, a specular reflection angle relative to the substrate surface is defined as an angle at which the incidence angle xcfx890 and the reflection angle xcfx89 are the same.
As shown in FIG. 6, for a specular reflection angle of 30xc2x0, the reflector 51 has a relatively good reflectance in the range of 15xc2x0xe2x89xa6xcfx89xe2x89xa645xc2x0.
The above-described reflector 51 of the known art exhibits relatively good reflectance over a relatively wide angle range due to the concave portions. However, as shown FIG. 6, reflectance at 30xc2x0, which is the specular reflection angle, is relatively low compared with two peaks at 15xc2x0 and 45xc2x0. Accordingly, reflection characteristics of the reflector 51 are such that although relatively good reflectance is ensured for a relatively wide range, brightness is reduced in the specular reflection angle.
However, when display units installed in devices such as notebook computers, desk calculators, watches, etc., are viewed, the direction of a light source (incidence angle) and a viewing angle of a user who receives reflection light (reflection angle) are normally in a specific range. Accordingly, it would be more convenient for the user to provide a display which not only appears bright in a wide area but also exhibits especially high reflection intensity in a specific direction.
In addition, in the case in which the above-described reflector, which appears bright over a wide viewing area, is used in liquid crystal displays having a backlight, a problem exists in that light emitted from the backlight is diffused too widely at the surface of the reflector and light emitted in the specular reflection angle, which is the angle at which a user normally views the display, is reduced.
In order to solve the above-described problems, an object of the present invention is to provide a reflector which exhibits good reflectance over a wide angle range and especially high reflectance in a reflection direction in a desired range, especially in a direction shifted from a direction of specular reflection, and which prevents light emitted from a backlight from being diffused too widely. In addition, it is also an object of the present invention to provide a reflective liquid crystal display which uses the reflector so that it appears bright over a wide viewing area and exhibits moderate directionality in a normal viewing area.
In order to solve the above-described problems, the present invention provides a reflector including a substrate having a plurality of light-reflective concave portions on the surface thereof, each concave portion having a first vertical section and a second vertical section which pass through a deepest point of the concave portion.
The first vertical section has an internal shape defined by a first curve and a second curve, the first curve extending from one point on the peripheral edge of the concave portion to the deepest point of the concave portion and the second curve extending continuously from the first curve from the deepest point of the concave portion to another point on the peripheral edge of the concave portion, and the average of the absolute value of an inclination angle of the first curve relative to the substrate surface is larger than the average of the absolute value of an inclination angle of the second curve relative to the substrate surface.
The second vertical section is perpendicular to the first vertical section and has an internal shape defined by a shallow curve and deep curves formed at both sides of the shallow curve, the deep curves having a smaller radius of curvature compared with the shallow curve.
Although the direction of the first vertical section is not determined, the vertical section along the up-down direction or the front-back direction relative to an observer is preferably defined as the first vertical direction.
As described above, in the reflector of the present invention, a plurality of light-reflective concave portions are formed on the substrate surface, and each of the concave portions has a curved surface (concave surface). Accordingly, the reflector appears bright from a wide viewing area and has a light-diffusing characteristic so that back reflection is suppressed.
The internal shape of each concave portion along the first vertical section is defined by the first curve and the second curve which are connected to each other at the deepest point. The first and the second curves are formed such that the average of the absolute value of the inclination angle of the first curve relative to the substrate surface is larger than the average of the absolute value of the inclination angle of the second curve relative to the substrate surface. More specifically, the inclination of the first curve is relatively steep and the inclination of the second curve is relatively gentle, and the second curve is longer than the first curve.
Accordingly, the amount of light reflected by the surface at regions around the second curve is larger than the amount of light reflected by the surface at regions around the first curve. More specifically, luminous flux density of reflection light in the direction of specular reflection relative to the surface at regions around the second curve is increased. Accordingly, when the first curves in each concave portion are aligned in a specific direction (or in a plurality of specific directions), reflectance in the specific direction(s) can be increased over the entire region of the reflector.
In addition, internal shape of each concave portion along the second vertical section, which is perpendicular to the first vertical section, is defined by the shallow curve and the deep curves formed at both sides of the shallow curve, the deep curves having a small radius of curvature. Accordingly, reflectance in the direction of specular reflection can be increased. Preferably, the deep curves are formed symmetrically across the shallow curve.
As a result, the overall reflection characteristics in the first vertical section are made such that peak reflectance is obtained at about the specular reflection angle and reflectance in the direction in which light is reflected by the surface at regions around the second curve B is increased. More specifically, reflection characteristics in which reflection light is moderately condensed in a specific direction without reducing the intensity of reflection light in the direction of specular reflection can be obtained.
According to the present invention, the concave portions are preferably formed such that the first vertical sections and the second vertical sections of each concave portion are aligned in the same direction and the orientations of the first curves in each concave portion are the same. More specifically, the orientations of the first curves in each concave portion are preferably made the same, and the orientations of the second curves in each concave portion are also preferably made the same.
In such a case, reflectance in the direction in which light is reflected by the surface at regions around the second curve B is increased over the entire region of the reflector. Accordingly, reflection characteristics in which reflection light is moderately condensed in a specific direction can be obtained.
In addition, according to the present invention, the inclination angle of the first curve relative to the substrate surface and the inclination angle of the second curve relative to the substrate surface are preferably substantially zero at the point at which the first curve and the second curve are connected to each other. In addition, preferably, when the inclination angle of the first curve is negative and the inclination angle of the second curve is positive, the inclination angle of the first curve is gradually increased from a negative value and the inclination angle of the second curve is gradually reduced from a positive value, and both the inclination angles of the first and second curves become substantially zero at the point at which the first and second curves are connected to each other.
Accordingly, the internal surfaces of each concave portion can be made smooth over the entire regions thereof, and reflectance in the direction of specular reflection can be prevented from being reduced.
Preferably, the concave portions are irregularly formed such that the depth thereof varies in the range of about 0.1 xcexcm to 3 xcexcm.
When the depths of the concave portions are less than about 0.1 xcexcm, sufficient light-diffusing effect cannot be obtained. When the depths of the concave portions exceed about 3 xcexcm, the thickness of the substrate, which must be larger than the depths of the concave portions, becomes too large and leads to problems in the manufacturing process and deterioration in product quality. Further, because a moirxc3xa9pattern occurs due to light interference when the concave portions are formed regularly, by forming the concave portions with various depths the emergence of a moirxc3xa9pattern is avoided. In addition, the reflection light is prevented from converging too sharply at a predetermined viewing angle and the amount of reflection light varies smoothly in the viewing area.
Preferably, the concave portions are irregularly arranged next to each other.
When the concave portions are formed separately, regions at which specular reflection occurs are increased since the regions between the concave portions are flat, and sufficient light-diffusing effect cannot be obtained in the limited pixel area. Accordingly, the concave portions are preferably formed next to each other. In addition, the concave portions are preferably formed irregularly since the moirxc3xa9pattern appears when the concave portions are formed regularly.
The present invention also provides a reflective liquid crystal display which includes one of the above-described reflectors. Preferably, the concave portions are formed such that the first vertical sections and the second vertical sections of each concave portion are aligned in the same direction and the orientations of the first curves in each concave portion are the same, and the reflector is installed such that the first curves are disposed above the second curves in each concave portion when viewed by an observer.
When the first curves are disposed above the second curves in each concave portion when viewed by the observer, external light, which is mainly incident from the upper side, can be reflected in a direction shifted toward the normal of the substrate surface from the lower side of the observer.
In addition, since external light, which is mainly incident from the upper side of the observer, is efficiently received at regions around the second curves, the amount of reflection light is increased over the entire region.
In addition, the amount of light reflected in the direction of specular reflection can be increased due to the reflection at the shallow curve in the second vertical section.
Accordingly, the amount of light reflected toward the eyes of the observer is increased, and a reflective liquid crystal display which appears bright from the viewpoint of the observer can be obtained.
The present invention also provides a reflector in which peak reflectance is obtained at about the specular reflection angle and an integrated value of reflectance in a reflection-angle range smaller than a specular reflection angle with respect to the substrate surface is different from an integrated value of reflectance in a reflection angle range larger than the specular reflection angle.
Accordingly, when a normal viewing angle of the observer is displaced from the direction of specular reflection relative to the substrate surface, a reflector in which light is mainly reflected in the direction of the normal viewing angle without reducing the amount of reflection light in the direction of specular reflection can be obtained.
The present invention also provides a reflective liquid crystal display which includes a reflector in which peak reflectance is obtained at about a specular reflection angle and an integrated value of reflectance in a reflection-angle range smaller than a specular reflection angle with respect to the substrate surface is different from an integrated value of reflectance in a reflection-angle range larger than the specular reflection angle. The reflector is installed such that the reflection-angle range corresponding to the larger of the integrated values of reflectance is disposed at the upper side of the specular reflection angle with respect to the substrate surface when viewed by an observer.
According to the present invention, external light, which is mainly incident from the upper side, can be reflected in the direction shifted toward the normal of the substrate surface from the lower side of the observer.
Accordingly, when the reflective liquid crystal display of the present invention is used as a display for a mobile phone or a notebook computer, the amount of light reflected toward the eyes of the observer is increased, and a reflective liquid crystal display which appears bright from the viewpoint of the observer can be obtained.
As described above, according to the present invention, a reflector can be obtained which has a light-diffusing characteristic so that incident light is diffusely reflected and back reflection is suppressed over a wide viewing angle, and in which the amount of reflection light in the viewing-angle range in which the observer normally views the display is increased.
In addition, in a reflective liquid crystal display containing the reflector of the present invention, display surface appears especially bright when viewed in a specific viewing-angle range so that visibility is improved, and back reflection is suppressed over a wide viewing-angle range.